In situ retorting of oil shale to recover the liquid and gaseous carbonaceous values present in the shale has heretofore been proposed. One such arrangement is described in U.S. Pat. No. 3,661,423 assigned to the same assignee as the present invention. The in situ retorting process described in this patent involves forming a cavity in the oil shale formation in which the cavity is filled with oil shale particles. Air is brought in at the top of the cavity to sustain combustion of the top layer of the oil shale particles. The hot products of combustion pass downwardly through the lower layers of oil shale particles and are withdrawn at the bottom of the cavity. This heats the oil shale particles up sufficiently to drive off the liquid and carbonaceous values from the oil shale particles. The liquid values accumulate at the bottom of the cavity and the carbonaceous values are withdrawn along with the product gases through a pipe terminating adjacent the bottom of the cavity.
While the in situ recovery process described in the patent is effective in the recovery of oil from oil shale, it has been found that the flow of air and product gases down through the retort may not be evenly distributed over the cross-sectional area of the cavity. As a result, the burning rate may not be uniform and the retorting may not proceed as efficiently in some areas as others. As a result, the entire volume of oil shale particles may not be completely retorted, thereby greatly decreasing the overall efficiency of the retorting process.